1. Field of the Invention
The present invention relates to a non-contact IC card and, particularly, to the setting of a threshold level for detecting a received signal.
2. Description of the Related Art
FIG. 3 shows the structure of a conventional non-contact IC card. In the drawing, to a CPU 31 are connected a modulator 33 and a demodulator 34 through an UART 32, an input/output control circuit, a transmitter 35, and a receiver 36 being respectively connected to the modulator 33 and the demodulator 34. An antenna 37 is connected to the transmitter 35 and the receiver 36.
An external signal received by the antenna 37 is detected by the receiver 36, converted from an analog signal to a digital signal by the demodulator 34, and then input to the CPU 31 through the UART 32. The CPU 31 performs data processing and outputs a transmitting signal through the UART 32 as occasion demands. The transmitting signal is converted from a digital signal to an analog signal in the modulator 33 and then transmitted from the transmitter 35 through the antenna 37.
A threshold level is previously set in the receiver 36 so that the signal received through the antenna 37 is detected by comparing the level of the signal with the threshold level. For example, when an input signal S.sub.10 including data D.sub.11 and D.sub.12 is input to the receiver 36, if the receiver 36 has an excessively high threshold value V.sub.H, the data D.sub.11 and D.sub.12 cannot be detected, as shown by an output waveform S.sub.H in FIG. 4. While if the receiver 36 has an excessively low threshold value V.sub.L, there is the danger of detecting not only the true data D.sub.11 and D.sub.12 but also noise N.sub.11 and N.sub.12, as shown by an output waveform S.sub.L in FIG. 4. The receiver 36 is therefore required to have a threshold level corresponding to the transmission level of external equipment which transmits a signal to the IC card.
However, even if a threshold level corresponding to the transmission level of external equipment is set, there is a danger of errors if the IC card is used in a different environment such as a very noisy place. IC cards having the same function are sometimes used in combination with various external apparatus having different transmission levels for particular uses.
In such cases, it is necessary to redesign the receiver 36 so that a threshold level corresponding to the environment and conditions of use are set therein. There is therefore a problem in that much time and labor are required.